Systemic mastocytosis, a clonal myeloproliferative disease with variable clinical manifestations is assoicated in most cases with the D816V mutation in the c-kit gene. Although the majority of patients with systemic mastocytosis do not have peripheral blood eosinophilia, a subgroup does exist and is classified as systemic mastocytosis with eosinophilia (SM-eo). Reports of patients with SM-eo describe the presence of either the Kit D816V mutation or the FIP1L1/PDGFRA fusion oncogene. Numerous similarities between patients with FIP1L1/PDGFRA and KIT D816V-associated peripheral blood eosinophilia have caused confusion about the management and specifically the role of imatinib in the treatment of these patients. It is of paramount importance to distinguish these two groups of patients with pathologically similar, but molecularly and clinically distinct diseases. We compared the clinical, laboratory, and molecular features of 12 patients who met WHO criteria for systemic mastocytosis (including presence of the D816V kit mutation) and had associated peripheral eosinophilia with those of 17 patients with peripheral eosinophilia and the FIP1L1/PDGFRA fusion oncogene Based on these comparisons, a number of clinical features appeared to be of potential use in distinguishing these two disorders. To confirm and standardize features which appeared useful on inital data analysis, statistical methods were employed to test 21 possible risk factors for their ability to distinguish Kit and FIP1L1/PDGFRA-associated disease. Calculated risk factor scores were developed based on this analysis. Applying this risk factor based system, 16/17 FIP1L1/PDGFRA patients were classified correctly, with one patient neutral and all 12 Kit D816V SM-eo patients were classified correctly. Thirty four FIP1L1/PDGFRA patients in the literature were available for analysis, although all risk factors to create the score were not available for all patients. Despite this, 25/34 FIP1L1/PDGFRA patients were correctly predicted as FIP1L1/PDGFRA, 4/34 patients were neutral and 5/34 were misclassified as Kit D816V?associated SM-eo. These data suggest that the risk factor-based system presented in this study is useful in distinguishing imatinib-sensitive FIP1L1/PDGFRA-associated disease from imatinib-resistant Kit D816V-associated disease. [unreadable] The identification of KIT D816V mutation in patients with systemic mastocytosis (SM) has gained a major prognostic significance in the last several years, largely because of the availability of tyrosine kinase receptor inhibitors such as imatinib. Imatinib was shown to be ineffective in patients carrying KIT D816V mutation, but effective in cases displaying some other c-kit mutations. Therefore, it is of paramount importance to correctly identify SM patients with KIT D816V mutation. However, the reported frequency of KIT D816V mutation in SM patients is highly variable in the literature (30%-over 95%). It has been suggested that such variability is due to patient selection, sensitivity of the molecular methods used to detect the mutation or the source of the tested specimen (peripheral blood (PB) vs. bone marrow (BM) aspirate). So far, there was no systematic study comparing PB and BM mutational findings in SM patients. In this study, we have performed mutational analysis of both PB and BM samples in SM patients and compared the results with pathological, clinical laboratory and flow cytometric findings in patients with and without detectable c-kit mutation in PB. [unreadable] We have analyzed in parallel BM aspirates and PB from 55 patients who came to our clinic for evaluation of SM. After diagnostic workup, which included physical evaluation, measurement of serum tryptase level, histological and immunohistochemical study of BM biopsy, flow cytometric analysis of mast cells and mutational analysis by RT-PCR/RFLP, 46 of 55 patients were diagnosed with SM using WHO diagnostic criteria. Nine patients did not fulfill WHO diagnostic criteria for SM and all tested negative for c-kit mutation. Out of 46 patients diagnosed with SM, all but two patients (44/46; 95.6%) tested positive for KIT D816V mutation in the BM, but only 9/46 patients (19.5%) had the mutation detectable in the PB. No tested patients carried FIP1L1-PDGFRa fusion gene. 42/46 patients (91%) fulfilled major WHO pathological criteria for diagnosis of SM (i.e., dense mast cell aggregates in the BM biopsy). The other 9% had increased atypical spindle-shaped mast cells in the BM biopsy without dense aggregates. Flow cytometric analysis of PB showed no significant increase in circulating mast cells in patients with detectable KIT D816V mutation in PB (less than 0.01% mast cells on average). Comparison of patients with and without detectable PB mutation showed more extensive bone marrow involvement by mast cells in PB positive patients (average 45% vs. 15% of marrow involvement), higher average serum tryptase levels (266 ng/ml vs. 85 ng/ml) and higher average peripheral blood absolute eosinophil count (710 vs. 234). Flow cytometric analysis of BM mast cells revealed that 100% of KIT D816V positive patients had aberrant CD25 expression on mast cells. CD2 expression was more variable, but comparable in both groups of patents (67% vs.69%).[unreadable] Patients with advanced SM have a poor prognosis, and there is no mast cell ablative therapy available for most patients who carry an activating point mutation in the c-kit gene. We report results of a prospective study evaluating the safety, engraftment, and possibility of inducing a graft-versus-mast cell (GvMC) effect after allogeneic nonmyeloablative hematopoietic cell transplantation (HCT) from an HLA-identical sibling. Three patients with advanced SM were transplanted. All achieved complete donor T cell chimerism followed by clinical evidence for GvMC effect. However, all patients experienced disease progression with the longest response duration of 39 months. The GvMC effect can be observed after nonmyeloablative HCT with limited efficacy. Effective cytoreductive therapy prior to HCT may be required for long-term disease control and cure.